1. Field of the Invention
This invention relates to semiconductor resonant tunneling structures, and more particularly, to a resonant tunneling device which includes an indirect gap barrier layer disposed on a substrate having a different lattice constant than the lattice constant of the barrier layer to thereby create a biaxial tensile stress in the barrier layer which results in an improved peak to valley current ratio.
2. Discussion of the Prior Art
The field of semiconductor fabrication is an important and increasingly complex technology which is absolutely vital to the continued advancement of solid state electronic device technology. The wide range of semiconductors and the various electrical properties associated with each of the semiconductor materials provide circuit designers with much flexibility in designing devices with varying electrical properties.
Until recently it was very unusual for optoelectronic and high-speed solid state devices to be fabricated with heterojunction structures in which the unstrained lattice constants of the different materials were not approximately equal, and in turn equal to a conveniently available substrate. The lattice matching requirement severely limited the material selection to two principal systems; namely, Al.sub.x Ga.sub.1-x As-GaAs grown on GaAs substrates, and InGaAs-InAlAs grown on Inp substrates. Unfortunately optimum material parameters for many devices are obtained with material systems or alloy compositions that are not lattice matched to these or other available bulk substrates.
With the advancement of semiconductor technology, convenient and practical methods of fabricating devices with mismatched lattice constants led to various enhanced electrical and mechanical properties of the devices. One such electrical/mechanical effect is that of a biaxial stress created when two semiconductor materials with different lattice constants are adjacent each other. The article "Molecular-Beam Epitaxial Growth and Characterization of Strained GaInAs/AlInAs and InAs/GaAs Quantum Well Two-Dimensional Electron Gas Field-Effect Transistors", Seventh Molecular Beam Epitaxy Workshop, Cambridge, Mass., Oct. 20-22, 1986 teaches defect free strained layer epitaxy to change device characteristics of quantum wells. The increased freedom associated with mismatched lattice constant compositions allows for the optimization of certain properties of the particular structure such as conduction edge discontinuity and the electron effective mass.
U.S. Pat. No. 4,665,415 to Esaki et al. and assigned to the same assignee as the present invention, discloses a FET which has a conduction channel disposed within a layer of material comprising a group III-V compound and having a crystalline lattice structure which is stressed in two dimensions by means of epitaxial growth upon a thicker and rigid supporting layer comprising a different group III-V compound having a larger lattice spacing. The stretching of the layer having the conduction channel shifts the energy levels of holes therein to remove the degenerate state that exists, thereby elevating light holes to an energy level characterized by increased mobility. Basically, a two dimensional stress is utilized to increase hole mobility thereby increasing the horizontal transport of holes which in turn increases hole current.